The first electromagnetic counterpart to a gravitational wave source has been discovered and the ePESSTO team led one of the most extensive studies on optical and near-infrared evolution of the transient. LIGO-Virgo detected a strong signal from a binary neutron star inspiral and merger for the first time (GW170817).

This is a ground breaking result that opens up a new field in astrophysics by combining the study of light with gravitational waves. The ePESSTO observing team of Joe Lyman and David Homan were on the NTT as the counterpart was found in NGC4993. We took the critical classification spectrum on the night of 2017 Aug 18 at 23:20, 24hrs after the discovery (Lyman et al. GCN 21582) of the transient now known with the IAU Name AT2017gfo. Our spectrum, combined with the fast fading from our early, precise Pan-STARRS photometry (Chambers et al. GCN 21617), showed this really was an unprecedented transient discovery.

Combining photometry and spectra from the UV (provided by the Swift team), through the optical and near-infrared (Pan-STARRS, GROND and Boyden) with physical modelling we published a letter in Nature showing that this matches theoretical models of merging neutron stars that produce an optical and near-infrared transient known as a kilonova. Our models show, that if the power source is radioactive decay, it has a power-law slope of -1.2 ± 0.3, in good agreement with r-process. We identify caesium and tellurium features in the early spectra, and model the rapidly fading and reddening luminosity to determine mass and opacity of the ejecta. Many questions remain, including the opacity velocity and ejecta composition, and if one or two dynamical components are required. Further modelling of the full data set is needed and to aid this effort we release all our calibrated data here.

A new phase in the evolution of PESSTO has started. The “extended” Public ESO Spectroscopic Survey of Transient objects was approved as a 2 year Large Programme by the ESO OPC starting in April 2017. From Period 99 we were approved to continue our spectroscopic survey as ePESSTO. The operational mode of around 10N per month over 9 - 10 lunations during the year will continue and we will continue to provide immediate public classifications and classification spectra through Astronomer’s Telegrams, the IAU Transient Name Server and WiSeREP.

The scientific focus of ePESSTO has been adjusted to focus on the most exciting new transient populations being discovered. We are especially focused on the interesting area of nuclear transients, and understanding what causes the diverse range of high luminosity variations at the cores of galaxies. These include tidal disruption events, very luminous circum-nuclear supernovae and blue hyper-variables. Superluminous supernovae are still a major focus along with unusual transients situated far (tens of kiloparsecs) from their host galaxies. Gamma ray bursts and transients associated with high energy or radio detections and non-photonic triggers will also be a major line of investigation. With the discovery of gravitational waves by LIGO and high energy cosmic neutrinos by IceCube, the era of multi-messenger astronomy is here and ePESSTO is well placed to play a major role.

The new instrument for the ESO NTT, which will replace SOFI and EFOSC2 is the Son of Xshooter (SOXS, PI Sergio Campana). This is due for final design review in 2018 and we envisage ePESSTO bridging the gap from now until SOXS is delivered to ESO in 2020.